I always expect to need to up my storage game but every time I go out I realize I kinda don’t. The space behind and under the rear seats is actually very useful (don’t have JBL though). I also use the compartments built into the bed, but I have a longbed, so I have two of those. One has my recovery strap and D-rings and the smaller one has ratchet straps.
Bigger/additional items that I will only have with me for a camping or day wheeling trip will be thrown into the rear floor (no kids and usually no passengers back there), and then a lot of it in the bed using the OEM cargo divider.
But I am too interested in what other people do. I know if you are really serious about your storage game,
@DECKEDUSA has a badass solution. Of course, since you posted this in the DIY forum, I imagine you’re more interested in seeing DIY setups.